Switched-mode voltage converters or power converters are widely used in the field of electric power supplies. Voltage converters can be implemented as controlled voltage sources that provide a controlled output voltage from an input voltage, or can be implemented as controlled current sources that provide a controlled (mean) output current. The input voltage can be a DC voltage or an AC voltage. Voltage converters which receive an AC voltage are, for example, Power Factor Controllers (PFCs). A PFC is, for example, used for providing a DC link voltage from an AC main voltage.
To supply high output currents, a voltage converter can be implemented to include two or more converter stages connected in parallel. Each converter stage includes at least one inductive storage element and a switch which controls a current flow through the inductive storage element. The switch is driven by a pulse-width modulated (PWM) drive signal, wherein the output current of each converter stage is controlled by the duty-cycle of the PWM drive signal.
The switches in the converter stages are usually implemented as transistors, such as, for example, MOSFETs or IGBTs. MOSFETs are unipolar transistors that are available with a low on-resistance. However MOSFETs with a low on-resistance usually have a high output capacitance. Each time the MOSFET is switched off, electric charge is stored in the output capacitance.
Voltage converters can be operated in different operation modes. One possible operation mode is a continuous conduction mode (CCM), which is, for example, used for supplying high output currents. When a voltage converter or a converter stage is operated in the CCM, a current through the inductive storage element does not decrease to zero. The output capacitance of a MOSFET implemented in a converter that is operated in CCM causes high switching losses during turn on due to the energy stored in the output capacitance.
IGBTs are bipolar transistors that at low currents have higher losses than MOSFETs.
There is, therefore, a need to provide a voltage converter that has low losses for low output currents or low output powers as well as for high output currents or high output powers.